Heat dissipation system with boundary layer disruption

ABSTRACT

A heat dissipation system that includes a structure having a surface with a cavity recessed on the surface. A wing or channel causes a vortex to occur in the cavity. Destabilizers, such as projections or recesses are disposed on the sidewall of the cavity to disrupt the local surface boundary layer that forms in the cavity. Alternatively, a plurality of freely moving bead elements are disposed in the cavity to disrupt the local surface boundary layer. A cover can be included that prevents the bead elements from exiting the cavity.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contract No.N00173-07-C-2055 awarded by U.S. Naval Research Laboratory.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to heat dissipation systems that useforced air or other type of gas flowing over the surface of an object toremove heat from the object. The present invention also relates to heatdissipation systems that use ducted flow of air or other type of gas toremove heat from an object.

2. The Relevant Technology

As shown in FIG. 1, when a gas 100, such as air, flows over a smoothsurface 102, the velocity of the gas near the surface 102 is less thanthe velocity of the free stream 104. This creates a layer of lowervelocity gas, known as a viscous boundary layer 106, near surface 102.The thickness of the viscous boundary layer 106 is defined by the point108 where the gas velocity is 99% of the free stream velocity. Becauseof the lower gas velocity, less heat transfer can take place between ahot surface 102 and the gas 100, thus reducing the heat exchangeefficiency. As a result, there is also a layer of higher temperature gasthat is formed that is known as a thermal boundary layer 110. Thethickness of the thermal boundary layer 110 is defined by the point 112where the temperature of the gas is 99% of what it would be for the freestream flow 104. The thickness of the thermal boundary layer 110 may ormay not coincide with the thickness of the viscous boundary layer 106.

From the foregoing discussion, it is obvious that the thinner thethermal boundary layer 110, the steeper the temperature gradient andthus the greater the heat transfer rate. Thus, there are variousapproaches that have been taken to attempt to reduce the thermalboundary layer 110 to increase heat transfer.

In one approach, the thermal boundary layer 110 is disrupted ordestabilized by patterning the surface with crests, dimples, ordepressions. The patterning produces eddies or vortices on the surface,which provide more turbulent flow, thereby minimizing the thermalboundary layer that has formed on the surface. However, the eddy orvortex flows have their own local boundary layers with a correspondingreduction in local heat exchange efficiency. As such, while thepatterned surface yields a better overall heat exchange efficiency thana smooth surface, the efficiency is not as high as it could be due tothe local boundary layers.

Accordingly, what is needed are heat dissipation systems that not onlyminimize the surface boundary layer to increase heat exchangeefficiency, but also minimize the local boundary layers so as toincrease the local heat exchange efficiency, thereby providing moreefficient and greater overall heat exchange capabilities.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be discussed withreference to the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope. In the drawings,like parts are given like reference numerals.

FIG. 1 is a cross-sectional side view showing the various boundarylayers that can form against a surface over which a gas flows;

FIG. 2 is a side perspective view of a surface having cavities formedtherein according to one embodiment of the present invention;

FIG. 3 is a cross-sectional side view of one of the cavities formed inthe surface shown in FIG. 2;

FIG. 4 is a cross-sectional side view of the cavity depicted in FIG. 3,also showing the flow of gas through the cavity;

FIG. 5 is a graph showing how temperature varies with distance from thewall;

FIG. 6 is a top plan view of the cavity depicted in FIG. 3, showing theairflow direction and the thermal boundary layer;

FIG. 7 is a top perspective view of a model of the cavity andsurrounding surface;

FIG. 8 shows velocity field results obtained using the model of thecavity depicted in FIG. 7;

FIG. 9 is a cross-sectional side perspective view of the cavity depictedin FIG. 3, with various types of destabilizers formed in or projectingout from the cavity wall;

FIG. 10 is a top plan view of the cavity depicted in FIG. 3, showing theairflow direction and the thermal boundary layer with destabilizersformed in the cavity wall;

FIG. 11 is a top plan view of one embodiment of a recess formed in thecavity wall;

FIG. 12 is a top plan view of another embodiment of a recess formed inthe cavity wall;

FIG. 13 is a cross-sectional top view of a plurality of cavities eachfluidly coupled to a duct via a nozzle according to one embodiment ofthe present invention;

FIG. 14 is a cross-sectional side view of one embodiment of a heatdissipation system showing two cavities with bead elements and a ducttherebetween;

FIG. 15 is a cross-sectional side view of a portion of one of thecavities shown in FIG. 14 showing the ledge in greater detail;

FIG. 16 is a top view of one of the cavities shown in FIG. 14 having amesh cover disposed thereon, with the mesh top being partially removedto show beads disposed within the cavity;

FIG. 17 is a cross-sectional side view of an alternative embodiment of aheat dissipation system having two cavities and a duct therebetween;

FIG. 18 is a cross-sectional side view of another alternative embodimentof a heat dissipation system having two cavities and a ducttherebetween;

FIG. 19 is a top perspective view of one embodiment of a heat generatingstructure formed from a main block and a plurality of inserts;

FIG. 20 shows top and side plan views of the main block depicted in FIG.19;

FIG. 21 shows side and bottom plan views of the inserts depicted in FIG.19;

FIG. 22 is a top plan view of one embodiment of a cover that can be usedwith the heat generating structure depicted in FIG. 19;

FIG. 23 is a top plan view of one embodiment of a gasket that can beused with the heat generating structure depicted in FIG. 19; and

FIG. 24 is a top plan view of one embodiment of a plate that can be usedwith the heat generating structure depicted in FIG. 19.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For purposes of this application, a heat emitting object or structure isdefined herein as any type of object or structure that is capable ofemitting heat. This can include, for example, objects or structures thatthemselves generate heat, such as electronic components, engines,motors, generators, heating structures, and the like. Alternatively aheat emitting object or structure can also include any object orstructure that is attached to or otherwise placed adjacent to anotherobject or structure to transfer heat away from the other object orstructure. Examples of this include plates, coverings, heat sinks, andthe like. Objects or structures that do not of themselves generate heat,but absorb or otherwise become hot can also fit within this definition.For example, objects and structures that absorb energy by radiation orconvection can be considered heat emitting objects or structures.Examples of this can include solar energy absorbers, such as solarpanels and objects with heat absorbing paint, as well as radiators andother types devices that have a fluid flowing therethrough or thereby.It is appreciated that the foregoing list is only exemplary; otherobjects and structures can also qualify as a heat emitting object orstructure as long as the object can produce or transfer heat.

Embodiments of the present invention employ cavities on the hot surfaceof a heat emitting object to disrupt a surface boundary layer formed onthe surface. Vortex flows are generated in the cavities to aid inremoving heat from the object. Heat transfer efficiency within thecavities is enhanced by two mechanisms: moving the hot gas vortices awayfrom the hot wall of the cavity and disrupting the local boundary layerof the vortex at the hot wall. There are at least two types of heatdissipation system geometries with which the invention can be used. Thefirst is a free stream flow over the surface in which the cavities areformed. The second is a ducted flow through the body and in which thecavities are formed. Each of these geometries is described in moredetail below.

Depicted in FIG. 2 is one embodiment of a heat dissipation system 120incorporating features of the present invention. Heat dissipation system120 comprises a heat emitting structure 122 as defined above, having anouter surface 124. Recessed within the outer surface 124 are one or morecavities 126. Cavities 126 are positioned on a portion of outer surface124 over which a gas (denoted by arrow 128), such as air, is caused toflow by a means for producing the flowing gas 129. The cavities cause adisruption in the surface boundary layer of the outer surface 124, thusimproving heat transfer efficiency of the system.

The means for producing the flowing gas can include a conventional typeof flow generator, such as a fan, a blower, a jet, a ducted system, andthe like. Alternatively, a passive system can be used as the means forproducing the flowing gas. For example, a flow of air naturally causedby wind can constitute the means for producing the flowing gas. Asanother example, the means for producing the flowing gas can comprise asystem in which the heat emitting structure is moved through a calm airto generate the flowing gas. It is appreciated that the foregoingexamples are exemplary only and that other types of means for producinga flowing gas can alternatively by used.

Heat emitting structure 122 further comprises means for forming a vortexin the cavity, as will be discussed in more detail below. Although thediscussion below generally refers to a single cavity, it is appreciatedthat embodiments having a plurality of such cavities is generallyenvisioned. In general, the number of cavities 126 is only limited bythe size of the surface and the size of each cavity. For very smalldevices only a few cavities 126 may be present, whereas thousands oreven millions of cavities 126 may be formed on the surface 124 of verylarge devices.

Turning to FIG. 3 in conjunction with FIG. 2, cavity 126 is depicted asbeing generally cylindrically shaped with an open end of the cylinderdisposed at the surface 124. Other shapes having axial symmetry canalternatively be used, as detailed below. Cavity 126 is bounded by afloor 130 that constitutes the closed end of the cylinder and anencircling sidewall 132 extending from the floor 130 to the outersurface 124. As shown in the depicted embodiment, sidewall 132 isgenerally normal to floor 130 as it extends to surface 124 and encirclesa central longitudinal axis 134 so as to form the wall of the cylinder.Alternatively, sidewall 132 can be curved as it extends from floor 130to the outer surface 124. The encircling sidewall 132 intersects thesurface 124 at a perimeter edge 136 that forms a generally circularmouth 138 on surface 124, constituting the open end of the cylinder. Ingeneral, cavity 126 is configured so that the gas that enters cavity 126will do so near the perimeter edge 136 of mouth 138 and exit cavity 126near a center portion 139 of mouth 138, as described in greater detailbelow.

As noted above, although cavity 126 is generally depicted ascylindrically shaped, other shapes can alternatively be used. Forexample, cavity 126 can also be formed in the shape of an ellipsoid, atoroid, or the like (see, e.g., FIG. 18). In addition, instead of havinga circular mouth, an oval, polygonal, irregular, or other shape can beused. Also, the floor 130 can alternatively be slanted, curved, conical,irregular or have other shapes. Furthermore, the encircling sidewall 132can slant inward on either end, to make the cavity 126 substantiallyfrustoconically shaped. Other shapes can also be used. In general, anyshape can be used for cavity 126 as long as it allows a vortex to beformed within the cavity, as described below.

Furthermore, the size of cavity 126 can also vary, again as long as itallows a vortex to be formed therein. For example, in one embodiment,cavity 126 has a maximum diameter ranging from about 1 mm to about 15 mmand more commonly from about 3 mm to about 10 mm. In other embodimentsthe diameter can be up to 100 mm.

In one embodiment, cavity 126 has a maximum depth extending from theouter surface 124 of the structure 122 to the floor 130 in a range fromabout 0.5 mm to about 30 mm and more commonly from about 1 mm to about15 mm. In some embodiments, the ratio of the diameter of cavity 126 tothe depth is in a range between about 4:1 to about 1:2.

As noted above, heat emitting structure 122 includes means for forming avortex in each cavity 126. In one embodiment, depicted in FIGS. 2-4, awing 140 is provided for each cavity as the means for forming thevortex. Wing 140 has a first surface 142 and an opposing second surface144 extending laterally between a first end 146 and a second end 148 andprojecting from outer surface 124 of heat emitting structure 122 to atop edge 150. Wing 140 is disposed at or adjacent to the perimeter edge136 of cavity 126, and conforms to the same general curve as sidewall132 of cavity 126. As such, first surface 142 of wing 140 is generallyconcave laterally so as to be aligned with sidewall 132 to form a smoothtransition between sidewall 132 and wing 140. As shown in FIG. 2, eachwing 140 is positioned at least partially on the downwind side 152 ofits respective cavity 126. That is, wing 140 is positioned so that atleast a portion of the forced gas 128 will flow over the cavity beforereaching wing 140.

The size of wing 140 is dependent on the speed of the gas and the sizeof the cavity. The height of wing 140 can range from about 2 mm to about20 mm and more commonly from about 3 mm to about 10 mm. The lateralwidth can range from about 0.5 mm to about 25 mm and more commonly fromabout 2 mm to about 10 mm. The first and second ends 146, 148 of wing140 are depicted as projecting from outer surface 124 in a substantiallyperpendicular fashion. Wing 140 is also depicted as having asubstantially flat top edge 150. Other sizes and shapes are alsopossible. For example, wing 140 can have a rounded top edge 150 and theends 146 and/or 148 of wing 140 can taper up from outer surface 124.Also, wing 140 can be set a short distance away from cavity 126 so thatthe perimeter edge 136 is freely exposed between the wing 140 and thecavity 126. In one embodiment (see FIG. 9), the top edge 150 of wing 140curves in toward cavity 126 such that wing 140 is concave both laterallyand vertically. The first surface 142 can be semicircular, triangular,polygonal, cylindrical, irregular, or some other shape.

As particularly shown in FIG. 4, because of the positioning of wing 140on the downwind side 152 of cavity 126, the wing 140 creates a localpressure increase at the downwind edge 152 as the streaming gas flow 128encounters the wing 140. This pressure increase causes a portion of thegas 154 to flow into the cavity 126 and form a vortex 156.

In alternative embodiments, wing 140 can be placed in other locationsaround cavity 126. For example, instead of being on the downwind side152 of cavity 126, wing 140 can be formed elsewhere around cavity 126 onthe perimeter edge 136. Doing so would likely cause a decrease in theamount of pressure that builds up at the wing, but this can becompensated by making the wing higher and/or longer. Wing 140 can alsobe movable from one position to another around the cavity. Otheralternative positions are also possible as long as a vortex 156 iscreated in cavity 126.

The gas in the vortex 156 travels down the cavity 126 as it rotatesaround the cavity 126 near the cavity sidewall 132. When the gas reachesthe floor 130 of the cavity 126, the gas moves radially inward towardthe central longitudinal axis 134 of the cavity 126 and then movesaxially back up the cavity 126 toward mouth 138. The gas then exitscavity 126 through the center portion 139 of mouth 138, as shown byarrows 158. The dwell time of the gas in the cavity 126 is dependent onthe cavity depth, the gas flow rate, and how many turns a small gasvolume completes before exiting the cavity. The dwell time is alsorelated to the size of the wing 140, the orientation of the wing 140relative to the free stream gas flow 128, and the profile of the wing140.

The formation of a vortex aids in removing heat from the cavity 126. Itis known in the art that a gas when heated will expand in volume, andthus become less dense, all other variables being constant. This heatedgas volume with lower density than the ambient gas will move in thedirection opposite to the force field vector orientation. For example inthe Earth's gravitational field that is pointing toward the center ofthe earth, less dense hot air rises (i.e., moves opposite thegravitational field) while denser cooler air falls (i.e., moves in thesame direction as the gravitational field).

A vortex creates a centripetal force field vector pointing outward fromthe center of rotation of the vortex. As such, in a vortex, less densehotter gas will move toward the center of rotation of the vortex (i.e.,opposite the direction of the force field) while denser cooler gas willmove toward the outer portion of the vortex (i.e., in the same directionas the force field).

Thus, in the present invention, the vortex in the cavity 126 establishesa centripetal acceleration field directed outward from the center ofrotation 134. This centripetal acceleration force is represented in FIG.4 as arrow 160. Because of the centripetal force field created by thevortex 156, the hotter, less dense gas moves toward the center of thecavity (i.e. toward central longitudinal axis 134), while the cooler,more dense gas introduced into the cavity 126 from the free stream flowmoves toward the sidewall 132 of the cavity 126. As such, once the gaswithin the vortex 156 next to the sidewall 132 absorbs heat from thesidewall 132, the heated gas becomes less dense and moves away from thesidewall 132 and towards the center of rotation 134. The heated gas thenexits cavity 126 through center portion 139 of mouth 138. At the sametime, the denser cooler gas moves toward the sidewall 132 due to thecentripetal force 160 where the gas can then absorb heat from thesidewall 132 before also moving away from sidewall 132 toward center134. This cycle will continue for as long as the vortex 156 remainsformed within cavity 126.

Because the heated gas moves away from sidewall 132 after the gas hasabsorbed heat from the sidewall 132, the efficiency of the heat transferprocess is greatly increased. In fact, it has been found in some casesthat the formation of an eddy or vortex in a cavity can increase theheat exchange coefficient (or local Stanton number) by up to a factor ofthree (see, e.g., N. Syred et al, Journal of Turbomachinery, Vol. 123,pp. 609-613).

Turning to FIG. 5, as one would expect based on the foregoingdiscussion, the temperature of the gas within a vortex goes down as onemoves away from the center of the vortex, as shown in profile 170. In aperfect world the temperature of the vortex would continue to fall asyou approached the outer edges of the vortex, as shown in the dashedprofile 172. However, when the outer edges of the vortex are formedagainst a heated wall, as in the present invention, the temperature ofthe gas tends to drastically rise from a separation point x as you getclose to the wall, as shown in profile 174. This is because a localviscous boundary layer and corresponding thermal boundary layer 180 formon the wall 182 as the gas in the outer portion of the vortex flows pastthe wall, as shown in FIG. 6. As the vortex stabilizes, the viscousboundary layer can thicken and even extend to the center of the vortexafter only a few turns 184 of the vortex.

The thickness of the laminar boundary layer δ_(1am) as a function oftravel x and Reynolds number Re_(∞), away from the wall is typicallygiven by the relation:

δ_(1am)=1.72x·(Re _(∞) ,x)^(−0.5)  (1)

It is easy to see that this laminar boundary layer is the limiting heatexchange factor due to the low gas thermal conductivity (˜0.026 W/(mK)for air at room temperature). From equation (1) it is clear that even adisrupted boundary layer will recover after only a few millimeters oftravel to be a limiting factor again, at least within limited velocitiesand Reynolds numbers achievable with existing blowers.

Turning to FIG. 7, a typical velocity field was modeled using COMSOLMultiphysics® software for a 10 mm diameter hole 186 with a wing 188subjected to an air flow at 100 CFM flow rate. As shown in FIG. 8, theair enters the hole from the periphery of the hole (see 190) and exitsat the center (see 192). Near the viscous boundary layer the thermalgradient is large as the hot air mixes with cold flow. Near the centerof the hole the thermal gradient is small and of reversed sign due tothe centripetal force, which tends to separate cold air from hot. Thepoint where the thermal gradient is zero, is termed the separationpoint, which is shown in FIG. 5 as point x. The higher the air velocityis in the hole, the closer this separation point will be to the viscouslayer.

As noted above, the boundary layer can be minimized by disrupting ordestabilizing the layer. For example, as noted above, the cavities 126formed on outer surface 124 disrupt the boundary layer that forms onouter surface 124. In addition to disrupting the outer surface boundarylayer, the present invention includes means for disrupting ordestabilizing the local surface boundary layer 180 that forms betweenthe vortex 156 and the encircling sidewall 132 within the cavity 126.

Turning to FIG. 9, the means for disrupting can include thermal boundarylayer destabilizers disposed within cavity 126 or formed on sidewall 132of cavity 126. The thermal boundary layer destabilizers can be recessedwithin sidewall 132, such as destabilizers 200, 202, and 204, or projectout from sidewall 132, such as destabilizers 206 and 208.

The destabilizers can take many different shapes and sizes. For example,destabilizer 206 is a substantially box-like projection that projectsout from sidewall 132 and extends at least partially around sidewall 132at the same axial height. In contrast, destabilizer 208 is also abox-like projection that projects out from sidewall 132, but extendsonly a small amount around sidewall 132 and spans the entire axialheight of sidewall 132 from mouth 138 to floor 130. Destabilizer 202 issimilar in dimensions to destabilizer 206 except that destabilizer 202is recessed in sidewall 132 instead of projecting out from the sidewall.Destabilizer 204 also is recessed in sidewall 132, but comes together atthe ends to form a lens-like shape. Finally, destabilizer 200 isrecessed in sidewall 132 and similar to destabilizer 208, spans all theway from the mouth 138 to floor 130. It is appreciated that theforegoing destabilizers are exemplary only and should not limit thescope of the invention. Other types, shapes, and sizes can also be used.

Both projection and recessed types of destabilizers will cause adisruption in the vortex flow and thus increase thermal transferefficiency. However, because the projections 206 and 208 project intothe flow of the vortex, the projections 206 and 208 tend to slow the airdown and cause the vortex to be less efficient and slower. As notedabove, this tends to thicken the boundary layer 180. As such, a highervelocity vortex may be necessary when projections are used.

As shown in FIG. 10, recesses formed in the sidewall 132 tend to disruptthe boundary layer 180 and cause the thickness of the boundary layer 180to decrease. However, unlike projections, the speed of the vortex islargely unaffected, making the recesses generally more efficient thanthe projections. However, either projections or recesses can be usedwith the present invention. In the following discussion, recess 200spanning the entire height of cavity 126 will be discussed. However,this is by example only and should not limit the scope of the invention.

Turning to FIG. 11 in conjunction with FIG. 9, each recess 200 isbounded by a first wall 210 and a second wall 212 that intersect at acorner 214 recessed within structure 122. First and second walls 210 and212 each extend to the sidewall 132 so as to form a substantial “v”shape with an opening 216 being formed on sidewall 132. The first andsecond walls 210 and 212 respectively form a leading edge 218 andtrailing edge 220 on sidewall 132 that bound the opening 216. Theleading edge 218 is configured to correspond to the portion of therecess 200 over which the air in the vortex first flows. As shown inFIG. 9, each of the walls 210 and 212 extend from the floor 130 ofcavity 126 to the outer surface 124 of the heat emitting structure 122,although, as discussed above, this is not required.

In other embodiments, first and second sidewalls 210 and 212 do notintersect and instead extend to a back wall recessed within sidewall132.

For the most efficient disruption of the boundary layer 180 withoutaffecting the vortex flow, the first wall 210 forms a sharp angle α withsidewall 132 at leading edge 218, while the angle β formed between thesecond wall 212 and the sidewall 132 at trailing edge 218 can be a morelinear-like angle. For example, in some embodiments, angle α can rangefrom about 90° to about 135° with a range from about 90° to about 120°being more common. In the depicted embodiment, angle α is about 90°. Incontrast, angle β of trailing edge 220 can range from about 110° toabout 160° with a range from about 120° to about 150° being more common.In some embodiments, angles α and β can be substantially equal so thatthe vortex can travel either direction over recess 200. For example, inthe embodiment shown in FIG. 12, angles α and β are both about 120°.Other angles can also be used. Alternatively, sides 210 and 212 can becurved sides.

The size of recess 200 can vary depending on the size of the cavity 126and the speed of the air flow. For example, the depth of the recess 200(i.e., the distance between the opening 216 and the corner 214) canrange from about 0.1 mm to about 0.5 mm with about 0.3 mm to about 0.4mm being more common. The width (i.e., the distance between the leadingand trailing edges 218 and 220) can range from about 0.5 mm to about 2mm with about 1-2 mm being more common. Other values can also be used.

As noted above, the local boundary layer 180 formed in cavity 126 canquickly stabilize and thicken after a disruption. Because of this, aplurality of recesses 200 can be spaced around the cavity 126 tocontinually disrupt the boundary layer 180, as shown in FIG. 10. Similarto the sizing of recesses 200, the amount of recesses 200 within eachcavity 126 and the spacing between recesses 200 can also vary dependingon the size of the cavity and the speed of the air flow. Thus, althoughfour equally spaced recesses 200 are shown in FIG. 10, this is exemplaryonly; more or less recesses 200 can alternatively be used, and therecesses may or may not be equally spaced apart from each other. In oneembodiment recesses 200 are about 2-3 mm apart.

With a plurality of recesses 200 formed on sidewall 132, the localboundary layer 180 within cavity 126 is disrupted and minimized and theheat transferring capabilities are thus increased.

Turning to FIG. 13, an alternative means for forming a vortex in cavity126 is shown using a channel to carry the gas to the cavity 126. In thisembodiment, instead of using wings 140, a duct 230 is formed on outersurface 124 or within heat emitting structure 122. Duct 230 is boundedby a sidewall 232 that extends the length of the duct, allowing a gas tobe forced into and through the duct, as depicted by arrow 234. Aplurality of nozzles 236 each extends from the sidewall 232 of the ductto an inlet 240 disposed on the sidewall 132 or floor 130 of a differentcavity 126 so as to form a channel that fluidly couples the duct 230 tothe cavity 126. The nozzle 236 bounds a passage 238 through which thegas can flow from the duct 230 and into the cavity 126.

It is appreciated that duct 230 and/or nozzles 236 can be integrallyformed with structure 122, as in the depicted embodiment, or separatestructures that are attached to or otherwise affixed to structure 122.For example, duct 230 and/or nozzles 136 can be conventional pipes orother known devices that allow a gas to flow therethrough into thecavities 126. Other types of devices can also be used.

To form a vortex in cavity 126, the nozzle 236 is positionedsubstantially tangential to sidewall 132 so that the gas that passesthrough passage 238 will follow sidewall 132 around cavity 126, causingthe vortex 156 to occur. As noted above, the gas can enter cavity 126through sidewall 132 and/or floor 130 as long as the vortex is formed.As noted above, many different types of ducts 230 and nozzles 236 as areknown in the art can be used. The duct 230 and nozzles 236 are sized tobe able to deliver a desired forced flow into the cavities 126, also asis known in the art.

As noted above, the ducted flow approach can be used in place of thewings as the means for forming a vortex in cavity 126. As such, theducted flow approach can be used with the destabilizers discussed abovein one embodiment of the present invention. Alternatively, a differentmeans for disrupting or destabilizing the local surface boundary layer180 that forms between the vortex 156 and the encircling sidewall 132can be used with the ducted flow approach. In this embodiment, insteadof forming projections and/or recesses on or within sidewall 132, aplurality of freely floating bead elements are used to disrupt the localsurface boundary layer 180.

Turning to FIG. 14, similar to heat dissipation system 120, heatdissipation system 250 comprises one or more cavities 252 recessed onthe outer surface 124 of heat emitting structure 122. Cavity 252 issimilar in size and shape to cavity 126, and thus is bounded by a floor130 and an encircling sidewall 132 extending from the floor 130 to theouter surface 124. Unlike cavity 126, however, cavity 252 has an annularledge 254 formed where the sidewall 132 intersects the outer surface124, as shown in greater detail in FIG. 15. Ledge 254 is substantiallyparallel to outer surface 124 and a sidewall 256 extends from ledge 254to outer surface 124 so as to form a channel 258 that can receive acover, as described in more detail below. Although depicted as having asubstantially rectangular cross-sectional shape, channel 258 can be anyshape that will be able to receive a cover, as discussed below.

Returning to FIG. 14, as discussed above with regard to the ducted flowapproach, cavity 252 includes an inlet 240 that is formed on sidewall132 (as depicted) or on floor 130 through which a gas enters cavity 252to form the vortex. Inlet 240 is defined by a first edge 262 and asecond edge 264 that bound an opening 266. The opening 266 can extendall the way from floor 130 to ledge 254 or some smaller amount thereof.The opening 266 is sized so that bead elements, discussed below, cannotexit cavity 252 through the opening 266.

Turning to FIG. 16 in conjunction with FIG. 14, the means for disruptingor destabilizing the local surface boundary layer comprises a pluralityof bead elements 270 freely disposed within the cavity 252. Beadelements 270 are small (in relation to the cavity 252) regular orirregular shaped elements adapted to be moved around in the cavity 252by the flow of the gas. As noted above, a vortex creates a centripetalforce field vector pointing outward from the center of rotation of thevortex. Due to this centripetal force, as the gas flows through cavity252 to form a vortex 271, the bead elements 270 are forced toward thesidewall 132. As the gas flows over the bead elements 270, the beadelements 270 spin around the cavity, rotating and sliding along thecavity sidewall 132 and thereby disrupting the local surface boundarylayer. This disruption leads to significant improvements in local heatexchange coefficients.

The bead elements 270 comprise a main body 280 having an outer surface282 configured to be able to withstand the turbulent flow and frequentcollisions with the sidewall 132 and with the other bead elements 270.Bead elements 270 are sized to disrupt the local boundary layer 180(FIG. 6) within cavity 252, but still allow a substantially free flow ofgas in the vortex 156. Bead elements 270 can range from about 0.3 toabout 1.5 mm across with a range of 0.5 to 1.0 mm being more common.Other sizes can alternatively be used. Furthermore, bead elements 270within cavity 252 can all be the same size or can be of varying size.Bead elements 270 can be spherical, cylindrical, polygonal, irregular orany other shape.

The number of bead elements 270 within each cavity 252 is highlyvariable and depends on the size of the bead elements 270 and the sizeof the cavity 252. In some embodiments, the number of bead elements 270can range from about 5 to about 500 with a range of about 10 to about100 being more common. However, even having only a handful of beadelements 270 within each cavity 252 will cause heat exchange efficiencyto increase.

In general, bead elements 270 can be comprised of one or more of: afluoropolymer, ceramics, a refractory rubber, a metal, or a metal alloy.To withstand the beating that they take within cavity 252, bead elements270 can be made of one or more of Teflon, Rulon, Nylon, or elastomer.These materials tend to hold up well over an extended period of time ofwear. For example, it is anticipated that the life span of Rulon beadscan be measured in terms of decades. Alternatively, highly thermallyconductive ceramics or metal or alloys are used to even further improvethe thermal exchange between the structure 122 and the gas within thecavity 252.

In some embodiments, the expected temperature of the gas in the cavitycan be quite high, which can affect the type of bead elements 270 touse. For example some versions of Nylon have a relatively low meltingpoint around 190° C. and will begin to melt if the temperature withincavity 252 rises above that. Therefore, in those embodiments, aluminumor an aluminum alloy or a ceramic, such as AlN could be used for thebead elements 270. To increase longevity of the bead elements, ananti-wear coating can be incorporated on the bead elements 270 and/or onthe cavity sidewall 132. Of course aluminum or other metal or ceramiccan also be used to make the bead elements 270 that are used inembodiments where temperatures are expected to remain much lower than190° C.

To prevent the bead elements 270 from escaping cavity 252, a cover 272is provided for each cavity 252. Cover 272 comprises a retaining ring274 having a grid 276 attached thereto. Retaining ring 274 is a metal orother type of retaining material that is sized so as to fit within ledge254 at the mouth 138 of cavity 252. Grid 276 comprises a mesh or othertype of grid that is sized so as to allow the gas escaping the cavity252 to pass therethrough but prevent the bead elements 270 from passingtherethrough. In one embodiment the grid comprises a steel mesh having aspacing between grid members of about 0.5 mm. Other sizes canalternatively be used, so long as the spacing is less than the size ofbead elements 270.

As shown in FIG. 14, once bead elements have been placed within cavity252, cover 272 is positioned over cavity 252 by placing retaining ring274 so that it rests on ledge 254. Once cover 272 is thus positioned, aplate 278 can be attached to outer surface 124 to overlap retaining ring274, and thereby secure cover 272 in place. Plate 278 has openings cutout where the plate is above cavities 252 to allow the gas to escape thecavities. As shown in the depicted embodiment, plate 278 can also form aportion of the wall 232 of duct 230. A gasket can also be positionedbetween plate 278 and outer surface 124, if desired. The plate 278 canbe made of a metal, plastic, or other like material. In one embodiment,plate 278 is made of aluminum. In some embodiments, plate 278 isomitted.

Alternatively, cover 272 can be secured in place by a clip, adhesive, afastener, or other securing device known in the art.

The cover 272 discussed above is but one example of a cover that can beused with the present invention. Other grid covers, known in the art,can also be used. For example, the retaining ring 274 can be omitted, ifdesired, and cover 272 can be fastened directly to surface 124, with orwithout the ledge 254 being present. Furthermore, cavity 252 canalternatively be configured to allow cover 272 to be disposed part ofthe way down cavity 252, such as in an annular slot or the like. Otheralternatives can also be used.

Turning to FIG. 17, an alternative embodiment of a heat dissipationsystem 290 is depicted having an alternative cover 292. Heat dissipationsystem 290 is similar to heat dissipation system 250, except thatinstead of providing a separate cover 272 for each cavity 252, a singlecover sheet 292 is used to cover all of the cavities 252. Cover sheet292 comprises a mesh or other type of grid 294 similar to grid 276, butdoes not require a retaining ring. Instead, cover sheet 292 ispositioned between plate 278 and outer surface 124 of heat generatingstructure 122. As such, cover sheet 292 is retained in place overcavities 252 by plate 278. Although not required, a gasket 296 is alsopositioned between plate 278 and cover sheet 292. A gasket can also beplaced between cover sheet 292 and outer surface 124 either in place ofgasket 296 or in conjunction with gasket 296. Because a retaining ringis not required with cover sheet 292, no ledge is required to be formedon sidewall 132 at mouth 138 of cavities 252.

In some embodiments, a cover and grid are not used and mouth 138 ofcavity 252 remains uncovered. In these embodiments the bead elements 270remain within the cavity due to the shape of the cavity.

For example, turning to FIG. 18, an alternative embodiment of a heatdissipation system 300 is depicted that does not include a cover orgrid. Heat dissipation system 300 is similar to heat dissipation system250, except that instead of having cylindrical cavities 252, heatdissipation system 300 includes cavities 302 that are essentially atleast partially ellipsoid in shape. That is, instead of a substantiallylinear sidewall extending between floor 130 and mouth 138, sidewall 304curves out and away from the longitudinal axis 134 of cavity 302 betweenfloor 130 and mouth 138. As a result, the radial diameter of the cavityis larger at a central section 306 of sidewall 304 than at the floor 130or at the mouth 138.

As noted above, the outward-pointing centripetal force field vectorcaused by the vortex causes the bead elements 270 to move outward towardthe sidewall 304. Because of the larger radial diameter of the centralsection 306, the centripetal force also causes the bead elements 270 toremain generally against the central section 306 while at the sidewall304. The bead elements 270 do not move up to the mouth 138 because to doso would require the bead elements 270 to also move toward thelongitudinal axis 134, against the centripetal force. As a result, thebead elements 270 remain within cavity 302 without a cover. Of course acover can be used with this embodiment if desired.

Other cavity shapes can also be used to obviate the need for a cover,such as a toroid or other shape in which the radial diameter of thecavity at the central section is larger than the radial diameter of thecavity at the mouth or floor.

The heat dissipation systems described herein can be made of any type ofmaterial that has a high thermal conductivity. For example, heatdissipation systems according to the present invention can be made of ametal, a heat conducting ceramic, an alloy, or other material known inthe art. In one embodiment, aluminum or copper is used.

The heat generating structure can be formed from a single piece ofmaterial, or can be formed from separate pieces of material and attachedor otherwise secured together. FIGS. 19-21 disclose one embodiment inwhich a heat generating structure 310 is formed from separate materials.Heat generating structure 310 comprises a main block 312 and a pluralityof inserts 314 that, when assembled together, form the assembledstructure 310 depicted in FIG. 19.

As shown in FIGS. 20 and 21, main block 312 is formed so as to includetwo cavities 316 each extending upward from a floor 318 to a top surface320. Each cavity 316 is formed so as to bound portions of cavities 252,nozzles 236, and duct 230. A plurality of holes 322 are recessed withinfloor 318 within each cavity 316.

Each insert 314 comprises a body 324 having a sidewall 326 extendingfrom a bottom surface 328 to an opposing top surface 330. A post 332extends down and away from bottom surface 328. Post 332 is sized andshaped to fit within holes 322 of main block 312 in a tenon and mortisearrangement. Each insert 314 is sized and shaped so that when post 332is inserted into one of the holes 322, the insert 314 defines a portionof two of the cavities 252, a portion of the nozzles 236 correspondingto the two cavities, and a portion of the duct 230.

In the partially assembled state depicted in FIG. 20, two of the inserts314 have been mounted within cavity 316. This partially assembled stateshows how the inserts 314 are shaped and positioned to help form thecavities 252, the nozzles 236, and the duct 230. To finish the assembly,the posts 332 of the remaining inserts 314 are inserted into separateholes 322. Inserts 314 are secured to main block 312 by adhesive,welding, or any other manner known in the art. In an alternativeembodiment, the posts 332 extend up from floor 318 and the holes 322 arerecessed within the bottom surface 328 of the inserts 314.

Once all of the inserts 314 have been mounted and secured to main block312 to form heat generating structure 310, the cover, gasket, and platecan be mounted thereto. FIGS. 22, 23, and 24 respectively show a cover340, a gasket 342, and a plate 344 that can be used with the heatgenerating structure 310. Note that holes 346 are cut out of the gasket342 and plate 344 where the gasket and plate align with the cavities252. The cover 340, gasket 342, and plate 344 are placed over the top ofthe heat generating structure 310 and secured using screws, adhesive, orother known securing devices or methods known in the art.

As noted above, each of the foregoing heat dissipation systemembodiments can be used in various products. For example, variouselectronic components can have the heat dissipation systems describedabove formed on the surface thereof so that a forced flow or a ductedflow can be used to efficiently remove heat from the electroniccomponent. These electronic components can include, for example,integrated circuits (ICs), discrete components (e.g., resisters,capacitors, etc) printed circuit boards (PCBs), and the like. The heatdissipation systems can alternatively be formed in a plate or otherstructure and thereafter attached to the electronic component. Thepresent invention can also be used in other types of heat emittingdevices, such as engines, motors, generators or other devices. When usedwith larger devices, such as engines or generators, the cavities andcorresponding structures of the heat dissipation systems can be larger.For example, the diameter of the cavity 126, 252 can be up to about fourinches and the depth of the cavity can be up to about eight inches.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A heat dissipation system configured to dissipate heat using an airflow, the heat dissipation system comprising: a structure having asurface; a cavity recessed on the surface of the structure, the cavitybeing bounded by a floor and an encircling sidewall extending from thefloor to the surface, the cavity having a central longitudinal axisextending through the floor, the encircling sidewall intersecting withthe surface at a perimeter edge; means for forming a vortex in thecavity that encircles the central longitudinal axis; and means fordisrupting a surface boundary layer that forms adjacent to theencircling sidewall.
 2. The heat dissipation system recited in claim 1,wherein the means for forming a vortex in the cavity comprises a wingdisposed at or adjacent to the perimeter edge of the cavity andprojecting out and away from the surface of the structure.
 3. The heatdissipation system recited in claim 1, wherein the means for forming avortex in the cavity comprises a channel coupled to the cavity throughan inlet formed on the encircling sidewall or the floor of the cavity.4. The heat dissipation system recited in claim 1, wherein the cavityhas a maximum diameter of less than about 15 mm.
 5. The heat dissipationsystem recited in claim 1, further comprising: a second cavity recessedin the surface of the structure, the second cavity being bounded by afloor and an encircling sidewall extending from the floor to thesurface, the second cavity having a central longitudinal axis extendingthrough the floor, the encircling sidewall of the second cavityintersecting the surface at a perimeter edge; and means for forming avortex in the second cavity that encircles the central longitudinal axisof the second cavity; and means for disrupting a surface boundary layerthat forms adjacent to the second cavity.
 6. The heat dissipation systemrecited in claim 1, wherein the structure comprises an electroniccomponent.
 7. A heat dissipation system comprising: a structure having asurface over which a gas can flow; a cavity recessed on the surface ofthe structure, the cavity being bounded by a floor and an encirclingsidewall extending from the floor to the surface, the encirclingsidewall intersecting the surface at a perimeter edge; and a wingdisposed at or adjacent to the perimeter edge of the cavity andprojecting up and away from the surface of the structure, the wing beingconfigured so as to cause a portion of a gas to enter and form a vortexin the cavity when the gas passes over the surface.
 8. The heatdissipation system recited in claim 7, wherein the structure comprisesan electrical component.
 9. The heat dissipation system recited in claim7, further comprising one or more thermal boundary layer destabilizersdisposed within the cavity or formed on the sidewall of the cavity. 10.The heat dissipation system recited in claim 7, wherein the wing has aconcave surface that faces toward the cavity.
 11. The heat dissipationsystem recited in claim 7, further comprising a plurality of spacedapart cavities recessed on the surface, each cavity having a wingassociated therewith.
 12. A heat dissipation system comprising: astructure having a surface; a cavity recessed on the surface of thestructure, the cavity being bounded by a floor and an encirclingsidewall extending from the floor to the surface; and one or moredestabilizers disposed within the cavity or formed on the encirclingsidewall of the cavity so as to disrupt a thermal boundary layer thatforms against the encircling sidewall when a gas is passed through thecavity.
 13. The heat dissipation system recited in claim 12, wherein theone or more destabilizers comprise one or more recesses formed on thesidewall of the cavity.
 14. The heat dissipation system recited in claim13, wherein each of the one or more recesses extends from the floor ofthe cavity to the surface of the structure.
 15. The heat dissipationsystem recited in claim 13, wherein the one or more recesses comprises aplurality of radially spaced apart recesses formed on the sidewall ofthe cavity.
 16. The heat dissipation system recited in claim 13, whereineach recess is bounded by a first wall extending from the encirclingsidewall at a front edge, the angle between the encircling sidewall andthe first wall at the front edge being at least 135 degrees.
 17. Theheat dissipation system recited in claim 12, wherein the one or moredestabilizers comprises one or more projections projecting from thesidewall and into the cavity.
 18. The heat dissipation system recited inclaim 12, wherein the one or more destabilizers comprises a plurality ofbead elements disposed within the cavity and configured to move when thegas is passed through the cavity.
 19. The heat dissipation systemrecited in claim 12, wherein the cavity has a maximum depth extendingfrom the surface of the structure to the floor in a range between about0.5 mm to about 30 mm.
 20. A heat dissipation system comprising: astructure having a surface; a cavity recessed on the surface of thestructure, the cavity being bounded by a floor and an encirclingsidewall extending from the floor to the surface, the encirclingsidewall intersecting the surface at a perimeter edge that bounds amouth of the cavity; and a plurality of bead elements disposed withinthe cavity so as to disrupt any thermal boundary layer that formsagainst the encircling sidewall when a flowing gas is passed through thecavity.
 21. The heat dissipation system recited in claim 20, wherein thebead elements are comprised of: a fluoropolymer, ceramics, refractoryrubber, metal, or metal alloy.
 22. The heat dissipation system recitedin claim 20, wherein the cavity has a maximum diameter and a maximumdepth, the ratio of the maximum diameter to the maximum depth being in arange between about 4:1 to about 1:2.
 23. The heat dissipation systemrecited in claim 20, further comprising a cover positioned over themouth of the cavity, the cover being configured to allow a flowing gasto pass out of the cavity while preventing the bead elements fromexiting the cavity.
 24. The heat dissipation system recited in claim 23,wherein the cover comprises a mesh.
 25. The heat dissipation systemrecited in claim 20, further comprising an inlet formed on theencircling sidewall and/or the floor for delivering a gas to the cavity.26. The heat dissipation system recited in claim 20, further comprisinga plurality of spaced apart cavities formed on the surface of thestructure, each cavity having a plurality of bead elements disposedtherein.
 27. The heat dissipation system recited in claim 26, furthercomprising: a duct formed on or embedded in the structure; and aplurality of channels formed on the structure, each channel extendingfrom the duct to a corresponding one of the plurality of cavities.
 28. Amethod of conducting heat, the method comprising: passing a gas into acavity formed on a surface of a structure so that the gas forms a vortexin the cavity, the gas in the vortex flowing by means for reducing thelaminar flow boundary layer between the gas and a sidewall of thecavity, and the gas absorbing heat from the sidewall and exiting thecavity.
 29. The method according to claim 28, wherein: passing the gasinto the cavity comprises passing the gas over the surface of thestructure such that the gas forms a high pressure area at a wingextending away from the surface, the high pressure area causing the gasto enter the cavity and form the vortex; and the gas in the vortexflowing by means for reducing the laminar flow boundary layer comprisesthe gas in the vortex flowing by a plurality of grooves formed in theencircling wall.
 30. The method according to claim 28, wherein: passingthe gas into the cavity comprises passing the gas through a duct and anozzle, the nozzle being fluidly connected to the cavity via an inlet inan encircling wall and/or a floor of the cavity; and the gas in thevortex flowing by means for reducing the laminar flow boundary layercomprises the gas in the vortex flowing by a plurality of bead elementsdisposed in the cavity, the plurality of bead elements being preventedfrom exiting the cavity by a cover positioned at a mouth of the cavity.